This is the 2973rd most frequent Portuguese word.
rato
Mouse; rat.
In this sentence, 'rato' refers to the animal commonly known as a rat.
Eu vi um rato no porão ontem à noite.
I saw a rat in the basement last night.
Here, 'rato' is used metaphorically to describe someone who spends a lot of time reading or studying books.
Ele é um rato de biblioteca, sempre estudando livros.
He is a bookworm, always studying books.
In this context, 'rato' signifies a computer accessory, known in English as a mouse, highlighting its technological usage.